W. C. Simpson

Production of O2 on icy satellites by electronic excitation of low-temperature water ice

The signature of condensed molecular oxygen has been reported in recent optical-reflectance measurements of the jovian moon Ganymede, and a tenuous oxygen atmosphere has been observed on Europa. The surfaces of these moons contain large amounts of water ice, and it is thought that O2 is formed by the sputtering ofice by energetic particles from the jovian magnetosphere. Understanding how O2 might be formed from low-temperature ice is crucial for theoretical and experimental simulations of the surfaces and atmospheres of icy bodies in the Solar System. Here we report laboratory measurements of the threshold energy, cross-section and temperature dependence of O2 production by electronic excitation of ice in vacuum, following electron-beam irradiation. Molecular oxygen is formed by direct excitation and dissociation of a stable precursor molecule, rather than (as has been previously thought) by diffusion and chemical recombination of precursor fragments. The large cross-section for O2 production suggests that electronic excitation plays an important part in the formation of O2 on Ganymede and Europa.

DISSOCIATIVE ELECTRON ATTACHMENT IN NANOSCALE ICE FILMS : THICKNESS AND CHARGE TRAPPING EFFECTS

The yield and kinetic energy (KE) distributions of D− ions produced via dissociative electron attachment (DEA) resonances in nanoscale D2O ice films are collected as a function of film thickness. The 2B1, 2A1, and 2B2 DEA resonances shift to higher energies and their D− ion yields first increase and then decrease as the D2O films thicken. The D− KE distributions also shift to higher energy with increasing film thickness. We interpret the changes in the DEA yield and the D− KE distributions in terms of modifications in the electronic and geometric structure of the surface of the film as it thickens. A small amount of charge build-up occurs following prolonged electron beam exposure at certain energies, which primarily affects the D− KE distributions. Charge trapping measurements indicate that an enhancement in the trapping cross section occurs at energies near zero and between 6 and 10 eV.

Dissociative electron attachment in nanoscale ice films: Temperature and morphology effects

The electron-stimulated desorption (ESD) of D− ions from condensed D2O films is investigated. Three low-energy peaks are observed which are identified as arising from excitation of 2B1, 2A1, and 2B2 dissociative electron attachment (DEA) resonances. A fourth, higher energy feature is also seen in the D− yield which is likely due to the formation of a transient anion state that dissociates and/or decays into a dissociative excited state. The energies and ion yields of the resonances vary with the temperature and morphology of the D2O film. Below 60 K, the work function of the ice films changes with temperature and the DEA resonances shift in energy. The D− ESD yield generally increases with temperature, but it deviates from this trend at temperatures corresponding to structural phase transitions in ice. The (2B1) D− temperature dependence is remarkably similar to that observed for the ESD of low-energy D+ ions from D2O ice, even though the two originate from different electronic excitations. These results ...

Stimulated desorption of Cl+ and the chemisorption of Cl2 on Si(111)-7×7 and Si(100)-2×1

The chemisorption of Cl2 on Si(111)-7 × 7 and Si(100)-2 × 1 and the mechanism for stimulated desorption of Cl+ from Si are studied with soft X-ray photoelectron spectroscopy (SXPS) and photon-stimulated desorption (PSD). It is shown that Cl2 interacts with Si(111)-7 × 7 at room temperature to form mono-, di- and tri-chlorides, while primarily monochlorides are formed on Si(100)-2 × 1. These differences are explained in terms of the reconstructions of each clean surface. The stimulated desorption of Cl+ ions has a threshold at ~ 20 eV that results from a direct excitation of a Cl 3s electron to an unoccupied Cl antibonding level. No direct desorption of Cl+ ions is observed at the Si 2p edge. Differences between the mechanisms for F+ and Cl+ desorption from Si are discussed.

Electron-stimulated desorption of D− (H−) from condensed D2O (H2O) films

The electron-stimulated desorption (ESD) of D- and H- ions from condensed D20 and H20 films is investigated. Three lowenergy peaks are observed in the ESD anion yield, which are identified as arising from excitation of 2B1, 2A1 and 2B 2 dissociative electron attachment (DEA) resonances. Additional structure is observed between 18 and 32 eV, which may be due to ion pair formation or to DEA resonances involving the 2a~ orbital. The ion yield resulting from excitation of the 2B 1 resonance increases as the film is heated. We attribute the increase in the ion yield to thermally induced hydrogen bond breaking near the surface, which enhances the lifetimes of the excited states that lead to desorption.

Photon- and electron-stimulated desorption of O+ from zirconia

We present a study of the photon- and electron-simulated desorption (PSD and ESD) of cations from yttriastabilized cubic ZrO 2 (100) and undoped amorphous ZrO 2 surfaces. For both types of zirconia, O+ is the primary ionic desorption product. A weak threshold for ESD of O+ from yttria-stabilized cubic zirconia (YSZ ) is observed at ~26‐27 eV with a rapid rise above ~30 eV. The PSD threshold from YSZ is ~31±1 eV. This is essentially the same as the O+ ESD and PSD thresholds (~30±1 eV ) from undoped amorphous ZrO 2 surfaces. The PSD O+ kinetic energy distributions extend from 0 to ~7 eV with a peak at ~2 eV and are similar from both surfaces. A comparison of the ion threshold data with photoelectron spectra indicates that desorption of O+ is primarily initiated by excitation of the Zr(4p) core level. All of the evidence is consistent with a desorption mechanism, in which the O+ ions are produced and ejected from the surface via a multi-electron Auger decay process.

ROLE OF SURFACE STOICHIOMETRY IN THE CL2/GAAS(001) REACTION

The room‐temperature reaction of Cl2 with GaAs(001)‐4×6, ‐c(2×8), and ‐c(4×4) surfaces is studied with synchrotron soft x‐ray photoelectron spectroscopy. The chemical composition of the reacted surfaces is found to depend on the stoichiometry of the starting surface. In all cases, the reaction occurs stepwise, with Ga and As monochlorides formed prior to the dichlorides. The Ga‐rich surface is initially more reactive than either of the As‐rich surfaces and it forms more GaCl than the As‐rich surfaces, which instead form more AsCl. The sticking coefficient for chlorine on GaAs(001) decays exponentially with coverage. A contribution from Cl atoms comprising the surface dichlorides is identified in the Cl 2p core‐level spectra.

Soft x‐ray photoelectron spectroscopy study of the reaction of XeF2 with GaAs

The room temperature reaction of atomic F (via XeF2 vapor) with GaAs(110) wafers is studied employing synchrotron‐based soft x‐ray photoelectron spectroscopy. Both film growth and etching occur simultaneously in this reaction, as the GaAs substrate is consumed in forming a GaF3 film while the excess As is liberated. The intermediate reaction products GaF, AsF, and/or elemental As, as well as tricoordinate Ga and As atoms, are present at the GaF3–GaAs interface throughout the various stages of film growth.

Bonding geometries of F and Cl on Si(100)-2 × 1

Angle- and energy-resolved F+ and Cl+ electron-stimulated desorption distributions are collected from Si(100)-2 × 1 surfaces exposed at room temperature to XeF2 or Cl2. These distributions are fit to a model that accounts for ion-surface interactions in order to extract bond angle information. It is found that both F and Cl chemisorb on the dangling bonds of intact Si dimers, at an angle of ∼ 20° from normal along the [011] azimuth. By annealing a chlorinated surface above ∼ 400 K, some normally oriented SiCl bonds are generated. In addition to determining bonding geometries, quantitative information is obtained regarding the image-charge interaction and neutralization for F+ and Cl+ interacting with Si.

Room-temperature chlorination of As-rich GaAs (110)

As‐rich GaAs (110) is prepared by ion bombardment and annealing, followed by chlorination and reannealing. The surface is then reacted at room temperature with Cl2 gas and examined with soft x‐ray photoelectron spectroscopy of the Ga and As 3d core levels. After low exposures (<5×104 L), the surface appears to passivate with half a monolayer of Cl adsorbed, primarily as AsCl. Following sufficiently large (≳5×104 L) exposures, however, the surface begins to etch, as indicated by the continuous uptake of chlorine and the formation of As and Ga chlorides. After the largest exposures, the distribution of As chlorides still favors the monochloride, whereas the Ga chlorides favor GaCl2. It is proposed that the heavily reacted surface is covered with –AsCl–GaCl2 treelike structures. The addition of Cl to form GaCl3 from GaCl2 is identified as the rate‐limiting step in the overall etching reaction.